Modules are self-contained software units that consist of models, views,
controllers, and other supporting components. End users can access the controllers
of a module when it is installed in application. For these reasons, modules are
often viewed as mini-applications. Modules differ from applications in that
modules cannot be deployed alone and must reside within applications.

A module is organized as a directory which is called the base path of the module.
Within the directory, there are sub-directories, such as controllers, models, views, which hold controllers,
models, views, and other code, just like in an application. The following example shows the content within a module:

Each module should have a unique module class which extends from yii\base\Module. The class should be located
directly under the module's base path and should be autoloadable.
When a module is being accessed, a single instance of the corresponding module class will be created.
Like application instances, module instances are used to share data and components
for code within modules.

When creating controllers in a module, a convention is to put the controller classes under the controllers
sub-namespace of the namespace of the module class. This also means the controller class files should be
put in the controllers directory within the module's base path.
For example, to create a post controller in the forum module shown in the last subsection, you should
declare the controller class like the following:

Views in a module should be put in the views directory within the module's base path.
For views rendered by a controller in the module, they should be put under the directory views/ControllerID,
where ControllerID refers to the controller ID. For example, if
the controller class is PostController, the directory would be views/post within the module's
base path.

A module can specify a layout that is applied to the views rendered by the module's
controllers. The layout should be put in the views/layouts directory by default, and you should configure
the yii\base\Module::$layout property to point to the layout name. If you do not configure the layout property,
the application's layout will be used instead.

Your module may also declare commands, that will be available through the Console mode.

In order for the command line utility to see your commands, you will need to change the yii\base\Module::$controllerNamespace
property, when Yii is executed in the console mode, and point it to your commands namespace.

One way to achieve that is to test the instance type of the Yii application in the module's init() method:

To use a module in an application, simply configure the application by listing the module in
the modules property of the application. The following code in the
application configuration uses the forum module:

The modules property takes an array of module configurations. Each array key
represents a module ID which uniquely identifies the module among all modules in the application, and the corresponding
array value is a configuration for creating the module.

Like accessing controllers in an application, routes are used to address
controllers in a module. A route for a controller within a module must begin with the module ID followed by
the controller ID and action ID.
For example, if an application uses a module named forum, then the route
forum/post/index would represent the index action of the post controller in the module. If the route
only contains the module ID, then the yii\base\Module::$defaultRoute property, which defaults to default,
will determine which controller/action should be used. This means a route forum would represent the default
controller in the forum module.

The URL manager rules for the modules should be added before yii\web\UrlManager::parseRequest() is fired. That means doing it
in module's init() won't work because module will be initialized when routes were already processed. Thus, the rules
should be added at bootstrap stage. It is a also a good practice
to wrap module's URL rules with yii\web\GroupUrlRule.

In case a module is used to version API, its URL rules should be added directly in urlManager
section of the application config.

Within a module, you may often need to get the instance of the module class so that you can
access the module ID, module parameters, module components, etc. You can do so by using the following statement:

$module = MyModuleClass::getInstance();

where MyModuleClass refers to the name of the module class that you are interested in. The getInstance() method
will return the currently requested instance of the module class. If the module is not requested, the method will
return null. Note that you do not want to manually create a new instance of the module class because it will be
different from the one created by Yii in response to a request.

Info: When developing a module, you should not assume the module will use a fixed ID. This is because a module
can be associated with an arbitrary ID when used in an application or within another module. In order to get
the module ID, you should use the above approach to get the module instance first, and then get the ID via
$module->id.

You may also access the instance of a module using the following approaches:

// get the child module whose ID is "forum"$module = \Yii::$app->getModule('forum');
// get the module to which the currently requested controller belongs$module = \Yii::$app->controller->module;

The first approach is only useful when you know the module ID, while the second approach is best used when you
know about the controllers being requested.

Once you have the module instance, you can access parameters and components registered with the module. For example,

Modules can be nested in unlimited levels. That is, a module can contain another module which can contain yet
another module. We call the former parent module while the latter child module. Child modules must be declared
in the modules property of their parent modules. For example,

For a controller within a nested module, its route should include the IDs of all its ancestor modules.
For example, the route forum/admin/dashboard/index represents the index action of the dashboard controller
in the admin module which is a child module of the forum module.

Info: The getModule() method only returns the child module directly belonging
to its parent. The yii\base\Application::$loadedModules property keeps a list of loaded modules, including both
direct children and nested ones, indexed by their class names.

Since version 2.0.13 modules support tree traversal. This allows module
developers to reference (application) components via the service locator that is their module.
This means that it is preferable to use $module->get('db') over Yii::$app->get('db').
The user of a module is able to specify a specific component to be used for the module in case a different component
(configuration) is required.

The application database tables will be prefixed with main_, while all module tables will be prefixed with module_.
Note that configuration above is not merged; the modules' component for example will have the query cache enabled since that is the default value.

Modules are best used in large applications whose features can be divided into several groups, each consisting of
a set of closely related features. Each such feature group can be developed as a module which is developed and
maintained by a specific developer or team.

Modules are also a good way of reusing code at the feature group level. Some commonly used features, such as
user management, comment management, can all be developed in terms of modules so that they can be reused easily
in future projects.